Expert: Tom Whiting - 5/17/2009

Question
What keeps the rotational axis of the Earth largely in line with the celestial background?

It seems more intuitive that the Earth's rotational axis would stay in a fixed position relative to the Sun. How can stars so far away tether the Earth's rotational axis to stay in alignment with them while it orbits its star?


Answer
Hi Ben,
The rotation of a massive Earth is the same as a rotating gyroscope,
and it obeys the laws of same. A gyroscope always maintains it's
axis of rotation as fixed in space, irrespective of anything else. The distant stars have no idea that the Earth exists, so our view of the same sky is maintained all the time.  Even the sun has no effect on this spinning gyroscope...even if the sun somehow magically disappeared, the Earth would still continue to spin with it's axis maintaining the same orientation in space.

The only exception to this is the slight bulge at the equator
and the moon's influence, so our "gyroscope" has a slight imperfection, and that's why it precesses around a little circle once every 25,800 years...around a tiny circle called precession of the equinoxes, while still maintaining it's 23.5 degree tilt from
the vertical.  Kind of like the motion of a dying toy top that's
nearing the end of it's spinning, because it's not perfectly
balanced either. So our axis slowly rotates around, Polaris being
our North Star now, and will be again about 26,000 years from now.
Thuban (Alpha Draconis) was the north star when the pyramids were
being built some 5000 years ago, and will be our north star again
around 21000 AD. The area involved at the true geographic north
pole is about the size of a tennis court, but when projected out
into space produces a circle with a 47 (twice 23.5 degrees) degree
diameter, and the spinning Earth takes 25,800 years to complete
one precession rotation.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA USA

FOLLOW UP:
Our revolution around the sun DOES produce a slight reflex motion
in the nearby stars, called parallax. Each nearby star does go
through a tiny reflex circle in the sky each year, and the farther away, the smaller the reflex parallax circle.  But even with the closest stars, this parallax circle is far less than one arc-second of angle. This parallax angle was first discovered by Friedrich Bessell in 1838, finally proving that Galileo and Copernicus were correct and that the Earth does indeed orbit the sun. (Prior to that, the argument used against a sun-centered solar system was, Well, if the Earth orbits the sun, where is your parallax circle for the closer stars?  No star exhibits a naked-eye parallax. They didn't realize how really far away the stars are, and how very small, microscopic, even the largest parallax angle is.)
The nearby star that Bessell used was 61 Cygni, with a parallax angle of only 0.3 arc-second, giving the distance of 1/0.3 = about 3.4 parsecs, or about 11 light years.  You can google F. Bessell if
you want to read up on that famous discovery in 1838.
Clear Skies,
Tom